Method and device for synchronisation in ofdm

ABSTRACT

A method of synchronising an Orthogonal Frequency Division Multiplexed (OFDM) IEEE 802.11a data packet at a receiver is disclosed. The 802.11a data packet has a series of short training sequence (STS) symbols as a preamble. Cross-correlation at the receiver of the STS in the 802.11a packet PLCP preamble with the modified reference STS, that is circular shifted by eight samples, results in a main correlation peak but with reduced pre- and post-lobes. To locate the cross-correlation peak, a running second derivative of the cross-correlation function is performed. Peak selection employs a running comparison of the position and magnitude of all peaks in the intermediate neighbourhood of the local peaks. Following selection of a peak from within the cross-correlation function of the first STS in the PLCP preamble, both the position and magnitude of the first STS is compared to those of the second STS. Based on the two independent calculations, the start of the OFDM frame is estimated.

FIELD OF THE INVENTION

The present invention relates to synchronisation methods, particularlyalthough not exclusively for Orthogonal Frequency Division Multiplexing(OFDM) communication schemes.

BACKGROUND OF THE INVENTION

The IEEE 802.11 standard outlines the media access control (MAC) and thePhysical Layer (PHY) layer specifications for wireless LAN. Two majorvariants have emerged from the IEEE 802.11 specifications, namely the802.11b and 802.11a standards. More specifically, the IEEE 802.11bemerged from 802.11 as the “high rate” and Wi-Fi™ standard specifyingthe DSSS system to operate at 1, 2, 5.5 and 11 Mbps. The IEEE 802.11bcompliant devices operate in only the 2.4000 GHz ISM band between 2.4000and 2.4835 GHz, while IEEE 802.11a, hereinafter referred to as‘802.11a’, compliant devices operate at the higher 54 Mbs rate in the 5GHz band, which supports even higher data rates owing to theimplementation of Orthogonal Frequency Division Modulation (OFDM).802.11a specifies the PHY layer operating in the 5 GHz band using theorthogonal frequency division modulation (OFDM) scheme to modulate thedata. The orthogonal frequency division multiplexing scheme allows highdata rate transmission by dividing the data streams into a number oflower data rate streams and then transmitting each one on a separate subchannel. An OFDM signal consists of a sum of subcarriers that aremodulated by using Phase Shift Keying (PSK) or Quadrature AmplitudeModulation (QAM). The signals are generated by Fourier Transformations.

In order to demodulate the received signal, an 802.11a receiver has tosynchronise. An OFDM synchronisation method performs two main tasks.First, the method must derive an optimum sampling instance and hencedetermine the symbol boundaries to minimise the effects ofinter-subcarrier interference (ICI) and intersymbol interference (ISI).Secondly, the method estimates and corrects the carrier frequency offsetof the received signal to avoid ICI.

Owing to low receiver sensitivities there is a loss in the peakmagnitude, and multipath leading to distortion. The current inventionprovides for a solution to the optimum sampling instance for determiningthe symbol boundaries to minimise the effects of inter-subcarrierinterference (ICI) and intersymbol interference (ISI). This relates toone aspect of the synchronisation method.

A typical example of OFDM physical layer is the IEEE 802.11a standard,hereinafter referred to as ‘802.11a’. The PLCP (Physical LayerConvergence Procedure) preamble consists of pre-defined sets of OFDMsignals, which are intended for use in synchronisation. In accordancewith the 802.11 medium access control (MAC) protocol, the receiverconstantly scans the medium to establish whether or not it is busy orfree for transmission. The receiver uses the preamble as a referencesignature to establish the presence of a packet on air. Once thepreamble is recognised at the receiver, the synchronisation methodestablishes an appropriate sampling interval and the rest of the packetis demodulated and decoded by the modem PHY.

The skilled addressee would acknowledge that a sequence of 10 identicalsymbols as shown in FIG. 1, each constituting a pre-defined shorttraining sequence (STS), is transmitted as the preamble of a frame.Typically, the receiver identifies the reception of a packet by crosscorrelating the received signal block against the STS. The OFDM signal(t1, t2, . . . ) is generated using the standard equations that would befamiliar to a person skilled in the art. The correlation processproduces peaks for every identical short training sequence received. Byimplementing a threshold detector, it is possible to identify theposition of the periodic correlation peaks, and hence the detection ofan OFDM frame and the consequent symbol boundaries.

However, in wireless communications, the transmit signal is distorteddue to the composite of multipath fading and signal shadowing. Inaddition, the auto-correlation profile of the STS manifests notable preand post cursors either side of the correlation peaks, which obscuresthe correlation peaks required for synchronisation. In multipathconditions, the cross correlation of the received signal preamble withthe STS shows dominant peaks before the desired correlation peak.Consequently, a threshold based peak detector fails to isolate thecorrect peak and hence the symbol boundaries are incorrectly identified.Furthermore, frame start position is also incorrectly calculated.

The current invention addresses the problem of synchronisation based onshort training sequences for the IEEE 802.11a standard; however, theensuing solution is equally applicable to other OFDM schemes. Hence, toovercome the limitations of the methods for synchronisation of receiversknown to the skilled addressee, the current invention provides for asolution to the problems of synchronisation within any packet switchednetwork or system using short training sequences, wherein the PHY layerpackets are transmitted over air are synchronised at the receiver byexploiting the presence of a predefined packet preamble.

SUMMARY OF THE INVENTION

According to a first aspect of the current invention there is provided amethod of identifying a signature point within at least one receiveddigital data frame transmitted over a communication link, said receivedframe containing a plurality of predefined identification markers, themethod comprising the steps of generating a signal whose magnitude isdetermined by the correlation of a reference signal with first andsecond predefined identification markers in the received signal,comparing, with a threshold, at least a first and second magnitude ofthe signal when the gradient of the correlated signal reaches or crossesa predetermined value, storing the first and second signal magnitudes,and the corresponding first and second sample times thereof when saidfirst and second magnitudes exceed said running threshold; identifyingthe presence of the signature point within the data frame on the basisof the gap between the first and second sample times.

Advantageously, said predetermined value of said gradient is indicativeof a local maximum of the generated signal, and the gradient of thesignal is only measured for a specified number of correlation samples.

Preferably the method further comprising ceasing the aforementionedcomparing step if said first and second magnitude does not reach saidthreshold within a defined number of correlation samples.

Advantageously, the threshold is the average signal strength of saidgenerated signal over a predetermined number of correlation samples, andaverage signal strength is weighted by a scalar value.

Preferably, the method further comprising the calculating the start timeof the received data frame on the basis of the identified signaturepoint, synchronising the digital data frame in accordance with theidentified signature point, and determining the sampling point of thereceived signal on the basis of the identified signature point.

In a preferred embodiment the method further comprising, searchingwithin a number of correlator samples around the first and second sampletimes for a third magnitude that is substantially the same as the firstor second magnitude, the corresponding third sample time thereof beingstored, searching within a number of correlator samples around the firstand second sample times for a third magnitude that is substantially thesame as the first or second magnitude, the corresponding third sampletime thereof being stored, and recalculating the start time of thereceived data frame in accordance with the third sample time.

In a further preferred embodiment, the reference signal corresponds withone of said predefined identification markers, and the reference signalis derived from one of said predefined identification markers.

In a second aspect of the preferred invention, there is provided amethod of generating a signal indicating, in at least one receiveddigital data frame transmitted over a communication link, the presenceof at least one predefined identification marker, the method comprising,generating a first signal based on said predefined identification datamarker but circular shifted by a predefined amount; and generating anoutput signal based on the correlated product of said first signal withsaid received predefined identification marker.

Preferably, the generated version of said predefined identificationmarker is left circular shifted, and said predefined amount is 8samples.

In a preferred embodiment, an Application Specific Integrated Circuit(ASIC) is arranged to perform the steps comprised in the methods.

In a further embodiment, a computer program comprising software codeportions may perform the steps of the method, when said program is runon a processor.

Preferably, a storage medium contains the computer program forperforming the steps of the method. Further, it is preferred that acarrier signal containing the computer programs.

In a further embodiment of the present invention there is provided awireless local area network receiver comprising a receiver for receivingat least one digital data frame having a predefined digital datasequence contained therein, and a processor arranged to implement thecomputer programs.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described,by way of example only, in relation to the following figures in which

FIG. 1 shows a timing diagram of Physical Layer Convergence Procedure(PLCP) preamble;

FIG. 2 shows a typical functional blocks of wireless LAN applicationspecific integrated circuits (ASICs);

FIG. 3 shows a functional block diagram of an application specificintegrated circuit (ASIC) according to a preferred embodiment;

FIG. 4 shows a graph of the pre-lobes before the main correlation peakand post-lobes after the main correlation peak in the prior art;

FIG. 5 shows a graph of the complex valued STS vector according to thepresent invention;

FIG. 6 shows a graph of the complex valued derived reference STSaccording to the present invention;

FIG. 7 shows an example of a cross-correlation output graph according toa preferred embodiment of the present invention;

FIG. 8 shows a flow diagram of the synchronisation method in accordancewith the present invention;

FIG. 9 shows a graph of the magnitude of the cross-correlator output(|r_(y,STS)(n)|) according to the present invention;

FIG. 10 shows a graph which is an example of the packet detect routineaccording to one embodiment of the present invention;

FIG. 11 shows a graph of the operation of the SPS and FPS method onceP_(D) has been located according to the present invention; and

FIG. 12 shows a graph of the operation of the PFA in multipath with highreceiver sensitivity in accordance with the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION

The 802.11 standard determines the Medium Access Control (MAC) andPhysical Layer (PHY) Specifications for wireless local area networks(LANs). In the ensuing text, full descriptions of the 802.11 and 802.11awireless network standards are omitted, it being understood that thisforms part of the general knowledge of the skilled addressee.

Personal computers (PCs), laptops and other portable devices incorporateNetwork Interface Cards (NICs) into the system architecture, whichpermits it to be connected to the wireless network and to other suchdevices. The laptop can communicate via an access point (AP) with otherdevices within the wireless network, or the wired network in a basicservice set (BSS), using the distribution service of the distributionsystem (DS). The NIC of the laptop can interface with one or morenetworks, for example the 802.11b and 802.11a, either as a single modeor dual mode connection to an AP.

FIG. 2 shows the typical functional blocks of wireless LAN applicationspecific integrated circuits (ASICs) including antenna 100 for receivinga signal S_(m)(t) 110, an RF portion 105, an analogue to digital part120 for sampling and yielding a digital signal S_(d) (nT) 125 which isfed into the baseband integrated circuit (BBIC) 130.

The objects of the current invention are achieved by implementing twoprimary steps: a reference sequence for cross-correlating it with thereceived frame; and a low complexity peak find and search (PFS) methodfor calculating frame start value for a received packet.

One of the objects of the current invention is to provide for areference sequence for cross correlating the received frame, whichreduces the probability of a dominant peak arising as a result of theSTS autocorrelation precursors. The latter is the case if the receivedsignal is cross-correlated with the STS, as present in the transmitframe preamble.

Another object of the current invention is to provide for a lowcomplexity Peak Find and Select (PFS) method, which helps to identifythe correct correlation peaks. One of the advantages of the currentinvention lies with the PFS method's capability to converge on thecorrect peaks even if multipath has displaced the location of the STScorrelation peak to an unexpected position.

Referring to FIG. 3, the instructions necessary to implement theprotocols are held on application specific integrated circuits (ASICs),which includes the A/D converter and modem elements. The peak findmethod of the current invention is implemented in hardware within thebaseband integrated circuit (BBIC), wherein the converted signalS_(d)(nT) from the analogue to digital converter (A/D) 10 is sent to thesynchronisation block 14 for implementing the peak find and selectmethod of the current invention so as to calculate the packet startvalue for a received 802.11a packet. Thereafter the rest of thesynchronisation method uses this reference counter to perform furtherfunctions, for example, to allow automatic gain control (AGC) 18,followed by phase and/or frequency correction 22 and demodulation 26.

The methods adopted, by the skilled addressee, to identify the presenceof a valid packet is to continuously cross-correlate received signalsamples with the pre-defined STS. The magnitude of the cross-correlationfunction r_(ySTS)(.) is then observed for the presence of a validsignal. $\begin{matrix}{{{r_{y,{STS}}(n)} = {\sum\limits_{l = 0}^{N - n - 1}{{y(n)}.{{STS}^{*}\left( {l + n} \right)}}}};{n \geq 0}} & (1)\end{matrix}$where y(nT_(s)) is the baseband equivalent complex valued receivedsignal, T_(s)=40 MHz, and STS* is the complex conjugate.

Fundamentally, the process of timing recovery using STS involvescross-correlating the received signal with the STS at the receiver. Themagnitude of the cross-correlator output is then observed with thepotential of finding strong periodic correlation peaks. The latter wouldbe observed if the received signal were to be the PLCP preamble of avalid 802.11a packet, as shown in FIG. 1.

The magnitude of the STS autocorrelation shows the characteristiccorrelation peak after 32 samples. However, it shows notable pre-lobesbefore the main correlation peak and post-lobes after the maincorrelation peak, as shown in FIG. 4. The presence of these peaks is dueto the non-ideal correlation properties of the STS number sequence.Whilst it is easy to detect the main correlation peak, in reality theexistence of strong pre and post-lobes does restrict the degree offreedom available in discriminating the desired main peak from the rest.The problem is particularly acute given the presence of AWGN andmultipath distortion.

In order to contain these undesirable peaks, the present inventionprovides for a reference STS such that its cross-correlation with thereceived PLCP preamble will yield pre and post lobes of reducedmagnitude. This would help increase the margin of discriminating themain correlation peak from the rest. It is determined that a shiftedversion of the STS facilitates in yielding pre and post lobes of reducedmagnitude. The optimum reference STS is a circular shift of the STS by 8samples. Graphs of the complex valued STS vector and the derivedreference STS are shown in FIGS. 5 and 6 respectively.

In order to confirm this, cross-correlation outputs using STS (defaultsequence) and the revised STS are compared. In both cases, identicalPLCP preamble is cross-correlated with the default STS and then with theshifted version. Performance with and without multipath is noted. Anexample of a cross-correlation output graph of these trials is shown inFIG. 7. Clearly, the use of the altered STS to cross-correlate thereceived signal helps reduce the magnitude of the pre and post lobeswithout affecting the magnitude of the mid-lobes. This is observed forinstances with and without multipath. Therefore the received signal iscross-correlated with the revised STS. Consequently, equation (1) isrewritten as, $\begin{matrix}{{{r_{y,{STS}}(n)} = {\sum\limits_{l = 0}^{N - n - 1}{{y(n)}.{{STS}_{ref}^{*}\left( {l + n} \right)}}}};{n \geq 0}} & (2)\end{matrix}$where,STS_(ref)={STS₈,STS₉,STS₁₀, . . . , STS₃₂,STS₁,STS₂, . . . ,STS₇}  (2.1)where the shifted short training sequence in accordance with one aspectof the current invention is denoted STS_(ref), and STS*_(ref) is thecomplex conjugate thereof.

The object of the current invention is to provide for a method ofidentifying cross correlation peaks, selecting the most appropriateones, and using the position of the selected peaks to estimate the startof the OFDM frame and hence establish the symbol detection boundaries.In doing so, it also has to accurately detect the presence of an OFDMframe from the cross correlation profile.

There are two parts to this method, namely, peak find and peak select.The Peak Find method performs a running second derivative of the crosscorrelation function, in order to find local maxima.

The Peak Select method identifies the peak corresponding to the firstmultipath component from within the cross correlation peaks of the firstand second STS. Whilst the IEEE 802.11a standard facilitates 10 STSwithin the frame preamble, it is highly desirable to identify the startof the frame as soon as possible. The Peak Select method performs arunning comparison of the position and magnitudes of all peaks withinthe immediate neighbourhood of the local peaks. Following the selectionof a peak from within the cross correlation function of the first STS,it then compares it (both position and magnitude) to that of the secondSTS. Based on these two independent calculations, it estimates the startof the frame.

The current invention provides for a timing recovery method is to devisea routine that identifies and STS correlation peak due to the 1^(st)multipath component of channel impulse response. The strongest observedcorrelation peak is not necessarily the one corresponding to the 1^(st)multipath tap. Therefore, the method of the current invention identifiesthe most appropriate correlation peaks from within the plethora ofuncorrelated peaks, in the presence of AWGN. The Peak Find method (PFA)of the current invention is designed to identify and select the mostappropriate STS correlation peaks in order to derive optimum samplinginstance for any given received packet.

FIG. 8 shows a flow diagram of the synchronisation method in accordancewith the present invention. The received signal 300 is cross-correlatedwith the reference STS sample vector 306 within the cross-correlator 304in accordance with equation 2. The magnitude of the correlator output isthen fed into the PFA 308, wherein the data is processed in the localpeak search 312, STS peak search 316 and the fine peak select 320portions of the method. The PFA 308 uses the correlator output toestablish the start of a packet, if a valid 802.11a OFDM packet isreceived. It is scheduled to constantly search for STS correlationpeaks. And so until it finds a packet-start value, the method will go onrepeating itself for every correlator output. Subsequently, the packetstart value may be used as a counter to estimate the received STS samplenumber 324, calculate the received signal magnitude on an antenna, andconsequently, set the automatic gain control 328 (AGC).

The principal functionalities of the PFA are: to detect the presence ofthe PLCP preamble of a valid 802.11a packet; to search and identify themost appropriate STS correlation peaks; and to calculate the start ofthe packet in order to establish best possible sampling instance.

Once the PFA has calculated packet-start value for a received 802.11apacket, the rest of the synchronisation method uses this referencecounter to perform other functions such as antenna selection and AGCupdate is this part of the invention.

The PFA performs a search and select operation on the correlator output.It incorporates mechanisms to mitigate the effects of AWGN and multipathpropagation. The use of a reference STS to perform the cross-correlationfunction helps ensure sufficient degree of freedom for the PFA toidentify the correlation peaks instead of converging on the uncorrelatedpeaks. The latter referred to as a ‘false-trigger’ and it is highlyundesirable. The PFA is said to have false triggered when it convergesto uncorrelated peaks instead of the desired STS correlation peaks.

Referring to flow chart of FIG. 8, the PFA 308 consists of 3inter-working routines and each is nested within one another. The firstroutine is the Local Peak Search (LPS) 312, which identifies peakswithin a sliding window of 3 correlator outputs; thus it searches andmarks out the local maxima in the correlator output stream. All LPSpeaks P are elements of the set P.

Secondly, referring to FIG. 8, the STS Peaks Search (SPS) 316 aspect ofthe method uses the local peaks to detect the presence of a valid PLCPpreamble. If so, then it marks all subsequent local peaks as those dueto the received PLCP cross-correlating with the reference STS, and theyare referred to as STS peaks, P_(tn). Thereafter, the SPS identifiespeaks that are most likely to be due to the 1^(st) multipath componentof the channel impulse response. These are referred to as STS_MAX peaksand denoted as, P¹ _(tn). The SPS then implements an iterative test onthe magnitude and location of found STS_MAX peaks to establish a pair ofcorrelation peaks, each corresponding to the 1^(st) multipath component,following reception of STS#1 (i.e., t₁) and STS#2 (i.e., t₂) of the PLCPpreamble respectively. These two peaks are referred to as MAX_PEAKS anddenoted as, P¹ _(t1) and P¹ _(t2). Given that STS correlation peaks areobserved every 32 samples of the correlator output, the positions of P¹_(t1) and P¹ _(t2) is then used to estimate the start of the receivedpacket.

Thirdly, the Fine Peak Select (FPS) aspect of the PFA checks theposition and magnitude of P¹ _(t1) relative to P¹ _(t2) and also withinthe immediate neighbourhood of these two peaks.

For the sake of clarity, {P¹ _(t1), P¹ _(t2)}⊂{P¹ _(t1), P¹ _(t2) . . ., P¹ _(t10)} ⊂{P_(t1), P_(t2), P_(t10)} c P. Also, the FPS resideswithin the SPS and that resides within the overall LPS routine.

The ensuing text describes an embodiment of the LPS aspect of the peakfind and select method of the current invention. This is the outer mostloop of the PFA method. This routine searches for local peaks fromwithin the output of the cross-correlator (defined in equation (2)). Then^(th) correlator output is assumed to be a local peak if it satisfiesthe expression in equation (3) below. Both magnitude and position ofevery m^(th) found local peak is noted. It is assumed that all localpeaks P(m) are elements of a set P.P(m)=|r _(y,STS)(n−1)|, where |r _(y,STS)(n−2)|≦|r _(y,STS)(n−1)≧|r_(y,STS)(n)|  (3)

The graph in FIG. 9 shows the cross-correlator output (|r_(y,STS)(n)|)which specifically represents the magnitude of the cross correlationoutput and the identified local peaks {P}. It is clear that since(|r_(y,STS)(n)|)≧0, P in effect is the envelope of |r_(y,STS)(n)|. Notethat according to equation (3), the calculation of P needs at least n+1correlator outputs.

In addition to finding local peaks in the correlator output, the LPSalso includes some initialisation flags and counters that are necessaryfor the operation of other algorithms nested within it. The flag‘startPeakAvg’ is implemented as a binary variable and is by default setequal to 0. It is set equal to unity when L_(P) seconds have elapsedsince the every first output from the cross-correlator. This duration isknown as the ‘lead-time’. During the lead-time, all routines of the PFAare inactive except the LPS. Local peaks found during the time interval[0, L_(P)−1] are gathered and they are then used to calculate itsrunning weighted average. This is described in the next section.

A further aspect of the method of the current invention is the STS PeaksSearch (SPS) routine, which performs two distinct tasks. First, itdetects whether or not an 802.11a packet is received or not. Secondly,if it has found a packet, the SPS then identifies two most appropriatecorrelation peaks from the first two received STS, such that the chosenpeaks are due to the 1^(st) multipath tap of the channel impulseresponse. The SPS is also equipped with a time-out mechanism, whichensures that if the STS peaks are not found within a predefined numberof correlation outputs, and then the routine will break out of thesearch and instead revert back to detecting a packet. This ensures thatthe timing recovery methods are not stuck in an endless search onoccasions when it has falsely detected the presence of a packet.

A transceiver incorporating the method of the current invention while inthe receive mode, must assess the medium for the presence of a valid802.11 packet. The medium is scanned constantly to search for an 802.11adefined packet, and a packet is identified by its characteristic PLCPpreamble. Also given the opportunistic behaviour of the CSMA/CAprotocol, it is not possible to anticipate when a packet might betransmitted. Therefore, the receiver must constantly process signalmeasurements at the antenna in anticipation of a packet. In order to doso, receive signal samples are correlated with the reference STS. Thecorrelator output would exhibit distinct peaks for when the receivedsignal is the PLCP header of an 802.11a packet. The SPS routine exploitsthe magnitude and position properties of the PLCP correlation peaks inorder to calculate the start of the packet; also termed packet-start.

The packet detect routine of the SPS uses the local peaks to calculatetheir weighted running average, P_(av). The P_(av)(r) is the r^(th)calculated running average using all m previously accumulated localpeaks, P(m−1), P(m−2), . . . P(1). A pre-determined window, L_(P), isused to accumulate local peaks, P. The set of accumulated peaks duringthe lead-time L_(P) is denoted as P(L_(P)). The size of this window isdefined in units of time. After L_(P) seconds, a weighted average of theaccumulated peaks is calculated using the pseudo code, an example ofthis is given in the appendix. Also, a binary flag startPeakAvg is setto unity, and this implies that P_(av) will now be calculated. Theconstant Γ₁ is a positive scalar value and it weights the local peaks'average.

Once startPeakAvg=1 is set, the SPS then enters another routine toidentify the presence of a packet. This is achieved by comparing themagnitudes of the latest local peak with the most up-to-date value ofP_(av). The binary flag, pktDetect is set to unity, once a valid 802.11apacket is detected. The position and magnitude of the m^(th) local peakis marked if it happens to be the peak that indicates the presence of apacket. This is denoted as the packet detect peak, P_(D). An example ofthe pseudo code for the packet detect routine is given in the appendix.

After a packet has been detected (i.e., pktDetect=1), the SPS thenconsiders all local peaks as STS correlation peaks and they are markedaccordingly, P_(tn); where the subscript tn represents the 10 STSs. Thisis shown in FIG. 10, which is an example of the packet detect routine.The circled region in the figure corresponds to the local peak when itsvalue exceeds that of the P_(av). This implies that the magnitude ofthat particular latest local peak is considerably large in comparison topreviously accrued peaks, and that suggests a strong correlation notpreviously observed. The SPS then uses P_(tn) to search for the 1^(st)multipath component of the channel impulse response for STS#1 and STS#2,i.e., P¹ _(t1) and P¹ _(t2) respectively. All P_(tn), are indexed usingthe counter sm, and they are initialised in the packet detect routine.

Having established that the correlator outputs are due to received PLCPpreamble, the SPS then embarks on a peak search routine wherein itattempts to isolate the 1^(st) multipath component of the channelimpulse response for STS#1 and STS#2, i.e., P¹ _(t1) and P¹ _(t2)respectively. A weighted running average of P_(tn) is calculated over 16observed peaks (including the current one). This average is denoted as,P^(av) _(tn)(j). If the magnitude of a STS correlation peak is found tobe greater than the current value of P^(av) _(tn)(j) and it is alsogreater than a pre-defined constant ψ, then that peak is considered tobe a likely candidate 1^(st) multipath component of the channel impulseresponse for STS#n, and it is represented {circumflex over (P)}_(tn)¹(sp). All such candidate peaks {circumflex over (P)}_(tn) ¹(sp) arereferred to as STS_MAX peaks and they are indexed using the counter sp.

An overview of an exemplary SPS routine is given in the appendix whichcontains the pseudo code for the whole SPS routine. Notice that theshaded regions are the ‘Antenna Select’ and ‘AGC update’ functions ofthe synchronisation method, which were referred to earlier in thedescription and more specifically in FIG. 8, block 328; that are nestedwithin the overall timing recovery method. This is because thepacket-start value output from the timing recovery's Fine Peak Selectroutine is used as a reference counter to set appropriate timing epochs.Accordingly, when stsNumber=4, STS#4 is said to be received and hencethe ‘Antenna Select’ and ‘AGC update’ functions of the synchronisationmethod are then invoked.

Reliable timing recovery is attained by using only the first 2MAX_PEAKS, P¹ _(t1) and P¹ _(t2). Therefore the maximum value of theindex sp is set to 2. Given that STS#1 and STS#2 are the first two STSto be received, the SPS continuously searches for the two most likelypeaks {circumflex over (P)}_(t1) ¹(1) and {circumflex over (P)}_(t2)¹(2) such that they would pass 4 STS_MAX tests. These tests consider themagnitude and position of the likely peaks to decide if the two peaks orany one of them is a likely candidate MAX_PEAK, P¹ _(t1) or P¹ _(t2)respectively. Either {circumflex over (P)}_(tn) ¹(1) or {circumflex over(P)}_(tn) ¹(2) or both may be discarded at the end of the 4 tests.Discarded peaks are no longer considered and the method waits until anew {circumflex over (P)}_(tn) ¹(sp) is available and once againcandidate MAX_PEAKS are assembled.

The newly assembled pair of STS_MAX peaks {circumflex over (P)}_(tn)¹(1) and {circumflex over (P)}_(tn) ¹(2) is then used to perform the 4tests once again to confirm whether or not the pair is the desiredMAX_PEAKS, P¹ _(t1) and P¹ _(t2), or not. This process continues until apair of STS_MAX peaks {circumflex over (P)}_(tn) ¹(1) and {circumflexover (P)}_(tn) ¹(2) is found such that they pass the 4 tests. This eventis marked by setting the binary flag ‘twoPeaksFound’ to equal to unity.Once this is so set, the SPS stops searching for STS_MAX peaks{circumflex over (P)}_(tn) ¹(1) and {circumflex over (P)}_(tn) ¹(2). Atthis point, the SPS method is said to have converged on the 1^(st)multipath component of the STS correlation peaks for STS#1 and STS#2.

Examples of the 4 STS_MAX tests are defined in the appendix. Each testis appropriately invoked when:

packet detect peak P_(D) is a STS_MAX peak, i.e., {circumflex over(P)}_(tn) ¹(1);

difference in the positions of STS_MAX peaks {circumflex over (P)}_(tn)¹(1) and {circumflex over (P)}_(tn) ¹(2) is greater than 32;

difference in the positions of STS_MAX peaks {circumflex over (P)}_(tn)¹(1) and {circumflex over (P)}_(tn) ¹(2) is less than 32; and

difference in the positions of STS_MAX peaks {circumflex over (P)}_(tn)¹(1) and {circumflex over (P)}_(tn) ¹(2) is equal 32.

The Fine Peak Select (FPS) routine is then invoked on the pair{circumflex over (P)}_(t1) ¹(1) and {circumflex over (P)}_(t2) ¹(2) thathave successfully passed the 4 STS_MAX tests. The output of the FPS isthe packet-start value, and it refers to the position of the 1st sampleof the received packet. This value is then used as a counter toestablish sample timing within the received packet. Also, a binarystatus flag ‘syncFound’ is set to unity to suggest that synchronisationhas been achieved. By using derived packet-start value and subsequentSTS_MAX peaks {circumflex over (P)}_(tn) ¹ the SPS is able to establisha reference counter, stsNumber. This is updated periodically to mark thereception of STS#3, STS#4, and so on.

The SPS implements a time-out mechanism in order to prevent it fromendless searching for MAX_PEAKS. The latter may occur if the packetdetect routine incorrectly converged to a peak P_(D). This is typicalfor when received signal SNR is low (i.e., at low receiver sensitivity)wherein the margin for error is small. The time-out mechanism ensuresthat the binary lag ‘pktDetect’ is set back to zero if packet-startvalue is not found within A (delta) correlator outputs after P_(D). Thisis shown in the example of the SPS pseudo code in the appendix.

The Fine Peak Select (FPS) routine is the inner most loop of the PFAmethod (FIG. 8). It is invoked only if two peaks {circumflex over(P)}_(t1) ¹(1) and {circumflex over (P)}_(t2) ¹(2) are successfullyfound from the SPS routine. The FPS performs a final check to ensurethat the 1^(st) multipath component of the STS correlation is notignored if it does not happen to be a local peak.

Note that the STS correlation peaks do not exactly reflect the channelimpulse response. The combination of dominant pre-lobes and poor SNR maycause the desired 1^(st) multipath peak to be buried and not yield adistinct peak. It is therefore necessary to check within the immediateneighbourhood of the found MAX_PEAKS for correlation outputs that are ofsimilar magnitude. The FPS performs this check for both {circumflex over(P)}_(t1) ¹(1) and {circumflex over (P)}_(t2) ¹(2) peaks. If neededeither one or both of these peaks are replaced by a neighbouringcorrelation output that may not necessarily be a local peak. An exampleof the pseudo code of the FPS routine is given in the appendix.

The packet-start value (referred to as pktStart) is adjusted by anumber: pktDelay. The latter is a scalar value and is optimised by trialand error simulations. The PFA method is said to have converged once apacket-start value is derived. Following this, the binary status flag‘syncFound’ is set to unity and a reference counter stsNumber is setequal to 2; to imply that STS#1 and STS#2 are received.

FIG. 11 shows a graph of the operation of the SPS and FPS method onceP_(D) has been found. The first two STS_MAX peaks are identified andthen tested to confirm them as MAX_PEAKS. The circled region shows thelack of strong correlation value in the immediate neighbourhood of thetwo peaks, and so the FPS returns the SPS derived MAX_PEAKS as theconfirmed MAX_PEAKS.

The timing recovery method is assessed in response to differentconditions to understanding how the PFA and its constituent searchroutines operate to isolate the 1^(st) multipath tap of the channelimpulse response from within the correlator output. Examples hereininclude different channel types and receiver sensitivities.

FIG. 12 shows the operation of the PFA in multipath with high receiversensitivity. The time-dispersive property of the multipath channeldistorts the STS correlation output. It introduces additionalcorrelation peaks due to the multi-tap channel impulse response.However, the SPS routine of the PFA with its characteristic magnitudeand position tests ensures that the desired MAX_PEAKS are picked out inamongst the several STS correlation peaks. The FPS ensures that notnecessarily the strongest peak is selected but the one, whichcorresponds to the 1^(st) multipath component.

The current invention is applicable to any packet-based network orsystem using short training sequences, wherein the PHY layer packets aretransmitted over air are synchronised at the receiver by exploiting thepresence of a predefined packet preamble. Further, it will be apparentto the skilled person in the art that variations and modifications canbe made to the described embodiments without departing from the scope ofthe invention.

APPENDIX

The following is an exemplary pseudo code for calculating a weightedaverage of the accumulated peaks if startPeakAvg = 1 if P(L_(P)) ≧64${P_{av}(r)} = {\frac{\Gamma_{1}}{64}{\sum\limits_{i = {{P{(L_{p})}} - 64}}^{P{(L_{p})}}{P_{i}}}}$else if P(L_(P)) ≧32 and P(L_(P)) < 64${P_{av}(r)} = {\frac{\Gamma_{1}}{32}{\sum\limits_{i = {{P{(L_{p})}} - 32}}^{P{(L_{p})}}{P_{i}}}}$else P_(av)(r) = ξ end r = r +1 end

The following represents a pseudo code that may be used to summarize thepacket detect routine. if pktDetect = 0 and startPeakAvg = 1 if P(m) > ξand P(m) > P_(av)(r−1) pktDetect = 1 P_(D) = P(m) sm = 1, sp = 1 end endif pktDetect = 1 and m > 16 P_(tn)(sm) = P(m)${P_{tn}^{av}(j)} = {\frac{\Gamma_{2}}{16}{\sum\limits_{i = {m - 16}}^{m}{P_{i}}}}$j = j + 1 if sm ≧3 and sp ≦2if  P_(tn)(sm − 1) > P_(tn)^(av)(j − 1)  and  P_(tn)(sm − 1) > ψP̂_(tn)¹(sp) = P_(tn)(sm − 1) STS_MAX Test 1 STS_MAX Test 2 STS_MAX Test3 STS_MAX Test 4 Fine Peak Select routine sp = sp + 1 end end if(syncFound ≠1 and pktDetect = 1) and (m > pos[P_(D)] + Δ) pktDetect = 0end if syncFound = 1 and sp ≧3if  pos[P_(tn)(sm)] − pos[P_(tn)¹(sp − 1)] ∈ [30, 34]P_(tn)¹(sp) = P_(tn) sp = sp +1 stsNumber = stsNumber + 1 end switch(stsNumber) case (3) case(4) Select Antenna Ste AGC case(5) case(6)case(7) case(8) case(9) case(10) end end sm = sm + 1 end

The above is an exemplary representation of an overview of the SPSroutine.

The following represents pseudo code for STS_MAX test 1. if sp = 1  if(mag[{circumflex over (P)}_(m) ¹(sp)] ≦ α₁.mag[P_(D)]) and(pos[{circumflex over (P)}_(m) ¹(sp)] − pos[P_(D)] ≦ 34)   {circumflexover (P)}_(m) ¹(sp + 1) = {circumflex over (P)}_(m) ¹(sp)   {circumflexover (P)}_(m) ¹(sp) = P_(D)   sp = 2  end end

The following represents pseudo code for STS_MAX test 2. if (sp = 2) and(pos[{circumflex over (P)}_(m) ¹(sp)] − pos[{circumflex over (P)}_(m)¹(sp − 1)] > 32)  pkDist = pos[{circumflex over (P)}_(m) ¹(sp)] −pos[{circumflex over (P)}_(m) ¹(sp − 1)]  p1 = mag[{circumflex over(P)}_(m) ¹(sp − 1)] ; p2 = mag[{circumflex over (P)}_(m) ¹(sp)]  if (p2≧α₂.p1) and (pkDist ≦ 37)   {circumflex over (P)}_(m) ¹(sp − 1) ={circumflex over (P)}_(m) ¹(sp)   sp = 1  else if (p2 < α₂.p1) and(pkDist ≦ 37)   sp = 2   twoPeaksFound = 1  else if (p2 ≧α₂.p1) and(pkDist > 37)   {circumflex over (P)}_(m) ¹(sp − 1) = {circumflex over(P)}_(m) ¹(sp)   sp = 1  else (p2 < α₂.p1) and (pkDist > 37)  {circumflex over (P)}_(m) ¹(sp − 1) = {circumflex over (P)}_(m) ¹(sp)  sp = 1  end end

The following represents pseudo code for STS_MAX test 3. if (sp = 2) and(pos({circumflex over (P)}_(m) ¹(sp)] − pos[{circumflex over (P)}_(m)¹(sp − 1)] < 32)  pkDist = pos[{circumflex over (P)}_(m) ¹(sp)] −pos[{circumflex over (P)}_(m) ¹(sp − 1)]  p1 = mag[{circumflex over(P)}_(m) ¹(sp − 1)] ; p2 = mag[{circumflex over (P)}_(m) ¹(sp)]  if (p2≧α₃.p1) and (pkDist ≧29)   {circumflex over (P)}_(m) ¹(sp − 1) ={circumflex over (P)}_(m) ¹(sp)   sp = 1  else if (p2 < α₃.p1) and(pkDist ≧29)   sp = 2   twoPeaksFound = 1  else if (p2 ≧α₃.p1) and(pkDist < 29)   {circumflex over (P)}_(m) ¹(sp − 1) = {circumflex over(P)}_(m) ¹(sp)   sp = 1  else (p2 < α₂.p1) and (pkDist < 29)   if(pkDist < 6)    if (p2 > β₁.p1)     {circumflex over (P)}_(m) ¹(sp − 1)= {circumflex over (P)}_(m) ¹(sp)     sp = 1    else     sp = 1    end  else if (pktDist ≧6) and (p2 > p1)    if (p2 > β₂.p1)     {circumflexover (P)}_(m) ¹(sp − 1) = {circumflex over (P)}_(m) ¹(sp)     sp = 1   else     sp = 1    end   else if (pktDist ≧6) and (p2 ≦ p1)    if(p2 > β₃.p1)     {circumflex over (P)}_(m) ¹(sp − 1) = {circumflex over(P)}_(m) ¹(sp)     sp = 1    else     sp = 1    end  end end

if (sp = 2) and (pos[{circumflex over (P)}_(m) ¹(sp)] − pos({circumflexover (P)}_(m) ¹(sp − 1)]=32)  pkDist = pos[{circumflex over (P)}_(m)¹(sp)] − pos[{circumflex over (P)}_(m) ¹(sp − 1)]  p1 = mag[{circumflexover (P)}_(m) ¹(sp − 1)] ; p2 = mag[{circumflex over (P)}_(m) ¹(sp)]  ifp2 > γ.p1   {circumflex over (P)}_(m) ¹(sp − 1) = {circumflex over(P)}_(m) ¹(sp)   sp = 1  else   twoPeaksFound = 1  end end

The following represents pseudo code for STS_MAX test 4.

The following represents a pseudo code of the FPS routine. if (sp = 2)and (twoPeaksFound = 1) for (sp = 1 to 2)if  (P̂_(tn)¹(sp) < λ ⋅ r_(y, STS)(pos[P̂_(tn)¹(sp)] − 1))  and(r_(y, STS)(pos[P̂_(m)¹(sp)] − 1) > P_(tn)^(av)(j − 1))P_(tn)¹(sp) = r_(y, STS)(pos[P̂_(tn)¹(sp)] − 1) elseP_(tn)¹(sp) = P̂_(tn)¹(sp) end end${pktStart} = {{\frac{1}{2}\left\{ {\left( {{{pos}\left\lbrack {P_{tn}^{1}({sp})} \right\rbrack} - 32} \right) + \left( {{pos}\left\lbrack {P_{tn}^{1}\left( {{sp} - 1} \right)} \right\rbrack} \right)} \right\}} - {pktDelay}}$syncFound = 1 stsNumber = 2 end

1-14. (canceled)
 15. A method of generating a signal indicating, in atleast one received digital data frame transmitted over a communicationlink, the presence of at least one predefined identification marker, themethod comprising: generating a first signal based on said predefinedidentification data marker but circular shifted by a predefined amount;and generating an output signal based on the correlated product of saidfirst signal with said received predefined identification marker.
 16. Amethod as claimed in claim 15, wherein said generated version of saidpredefined identification marker is left circular shifted.
 17. A methodas claimed in claim 15, wherein said predefined amount is 8 samples. 18.(canceled)
 19. An Application Specific Integrated Circuit (ASIC)arranged to perform the steps of claim
 15. 20. (canceled)
 21. A storagemedium comprising software code portions, which, when executed, carryout the steps of claim 15.